Magnetic resonance imaging (MRI) has been employed for various applications (e.g., medical imaging) for some time. In medicine, combined use of MRI with radiation therapy and with radiation imaging have both been investigated. Such multi-modality systems provide significant advantages compared to single modality systems where a patient may need to be moved or transferred from one system to another system. Such transfers can be difficult and time-consuming, and they can compromise results by complicating image registration.
MRI in combination with a radiotherapy accelerator has been considered by Raaymakers et al. in an article “Integrating a MRI scanner with a radiotherapy accelerator: a new concept of precise on line radiotherapy guidance and treatment monitoring”. In this article, an MRI system is described having main magnet coils and gradient coils disposed out of the path of the therapeutic radiation. Although the RF coils of the MRI system are in the radiation path, they do not cause enough absorption heterogeneity to significantly degrade therapy. More specifically, the RF coils in this work had an equivalent Al thickness of about 2.3 cm, which apparently is sufficiently low for the therapy being considered.
Combination of MRI with radiation imaging is more demanding than combination of MRI with radiation therapy. More specifically, if conventional MRI RF coils are disposed in the radiation path of a radiation imaging system, undesirable coil artifacts will tend to be present in the radiation images. For this reason, when MRI is performed in combination with radiation imaging (as opposed to radiation therapy), all coils of the MRI system, including the RF coils, are typically disposed out of the radiation path. For example, U.S. Pat. No. 6,925,319 considers a split magnet MRI system having all MRI coils disposed out of the radiation path of an X-ray system.
Unfortunately, MRI performance can be undesirably degraded by a requirement to place the MRI RF coils outside the field of view of a radiation imaging system. For example, surface RF coils are often placed directly on a subject being imaged for maximum MRI image quality. Such a surface coil is in the field of view of any radiation imaging system that is directed to the same part of the subject as the MRI system, which is the situation of greatest practical interest. Thus conventional combined MRI and radiation imaging can oblige an undesirable choice among accepting reduced MRI image quality (by placing the RF coils out of the radiation system field of view), accepting RF coil artifacts in the radiation images (by placing the RF coils in the radiation system field of view), or moving the MRI RF coils to one position for MRI imaging and to another position (out of the field of view) for radiation imaging.
Accordingly, it would be an advance in the art to provide radiation imaging compatible MRI RF coils. It would also be an advance in the art to provide radiation imaging compatible MRI systems, either separately or in combination with a radiation imaging system.